This special issue addresses open research topics related to ground observations of the Earth’s magnetic field. It consists of papers based on talks given during the XVIIth IAGA Workshop on Geomagnetic Observatory Instruments, Data Acquisition and Processing that was held in Dourbes (Belgium) from 4 September to 10 September 2016. Ground magnetic observatories are important for geomagnetism. They provide fixed, long-term, high-quality global time series, thus helping the scientific community to understand the geophysical processes of solid Earth as well as the interaction of the Sun with Earth’s magnetosphere and ionosphere. Data users such as space weather, space physics, and geomagnetic field modelling communities always require better quality data from observatories for their applications. This workshop aimed to improve observatory practice especially when faced with new challenges like advanced instrumentation development, higher sampling rates, faster definitive data processing, real-time delivery, and remote deployment. The event was successful in bringing together about 90 attendees from 37 different countries, of which 21 were European. The 72 presentations covered topics such as instrumentation, observation techniques and processing, data transmission, and applications. We invite the workshop participants and other researchers to submit their results to this inter-journal special issue of Annales Geophysicae (ANGEO) and Geoscientific Instrumentation, Methods and Data Systems (GI).

Passive electromagnetic induction soundings in the far field live on the fact that natural electromagnetic field variations (their source signals) fulfill the plane-wave assumption. In the frequency range of geomagnetic pulsations some signals (upstream waves) do and others (field line resonances) do not meet this condition. The consequences for several branches of such soundings are shown. A consequence is that a remote reference site may be situated in another continent than the local site.

The long-term drift of magnetometers at geomagnetic observatories is calibrated by a non-magnetic theodolite. We propose a numerical method to evaluate such absolute measurements in a new, more general manner. It is more flexible and helps to identify and correct or discard erroneous measurements. We derive this method and give examples showing how it improves the quality and reliability of the calibrations parameters (the so-called baseline values) of an observatory magnetometer.

This work presents the progressive steps which led to the successful setup of such measurements at the new magnetic observatory in Choutuppal (CPL) of CSIR-NGRI, Hyderabad, India. Iterative tuning of the setup led to the generation of good quality data from 2016 onward. The processes of commissioning this setup in low-latitude conditions, with the aim of producing 1 s definitive data, and the characteristics of the data from this new instrument are presented here.

In magnetic observatory data, according to the INTERMAGNET definitive 1 s data standard, the fluxgate magnetometer self noise usually covers the natural signal for frequencies higher than about 30 mHz. We present a numerical method how to merge the data with induction coil data in order to drastically reduce noise and to fill the entire possible bandwidth with information on the earth magnetic field. In spectrograms we visualize interesting phenomena revealed with the method.

Despite its importance to a range of applied and fundamental studies, and obvious parallels to a robust network of magnetic-field observatories, long-term geoelectric field monitoring is rarely performed. The installation of a new geoelectric monitoring system at the Boulder magnetic observatory of the US Geological Survey is summarized. Data from the system are expected, among other things, to be used for testing and validating algorithms for mapping North American geoelectric fields.

Absolute magnetic measurements require the vertical and the geographic north as reference directions. We present here a novel system able to measure the direction of the magnetic field and of the vertical and true north. A design of a north seeker is proposed that takes into account sensor bias as well as misalignment errors. Different methods are derived from this model and measurement results are presented. A measurement test at high latitude is also shown.

We have compared two methods of performing Absolute observations of the Earth's magnetic field. The newer, Residual method was evaluated for use at USGS geomagnetic observatories and compared with measurements using the traditional Null method. A mathematical outline of the Residual method is presented, including more precise conversions of the Declination angles to nanoTeslas (nT). Results show that the Residual method is better than the Null method, especially at high latitude.

We report on the history and recent developments of the Tatuoca magnetic observatory in Brazil. This observatory is located close to the geomagnetic equator and within a region of strong main field dynamics. Starting from 2015, we have installed new instrumentation and a new datalogger system. In the paper, we also comment on the challenges of doing absolute measurements at the geomagnetic equator.

We present a novel method for calibrating magnetic observatories. We show how magnetometer baselines can highlight a possible calibration error. We also provide a method based on high-frequency automatic absolute measurements. This method determines a transformation matrix for correcting raw data suffering from scale factor and orientation errors. We finally present a practical case where covered data have been successfully compared to those coming from a reference magnetometer.

In this paper, we give some background on calibration and verification of our automatic DI-flux instrument and then compare the automatic absolute magnetic measurements
with the human-made and discuss the advantages and disadvantages of automatic measurements.

A modern geomagnetic observatory is expected to record geomagnetic data with high stability, high resolution, and high reliability. Also, geomagnetic observatories may be located in remote areas, requiring low power consumption and simple maintenance. Here, we present a new data logger system that was designed to meet these criteria. This system is based on a Raspberry Pi embedded PC and includes a modular C++ software package which can be adapted to specific observatory setups.

Noise is a common problem for the experiments or observations. Noise in the raw data of magnetic observatories has features. The article makes an attempt to give a review of this noise, using the data from some Russian and Indian observatories.

For regional studies and practical use regional-scale models of the geomagnetic field are necessary. The Bulgarian Geomagnetic Reference Field (BulGRF) for 2015.0 epoch and prediction up to 2020.0 is produced and presented in this paper. It uses data from Panagyurishte (PAG) observatory and the Bulgarian secular network. As a result, our model gives a set of six coefficients which allow the users to obtain the main field values using only the geographic coordinates and a simple calculator.

Data loggers of magnetic observatories use GPS receivers to provide accurate time stamping of recorded data. Typical sampling rate is 1 s. A failure of the GPS receiver can result in erroneous time stamps. The observatory of Lanzhou, China, accumulated a lag of 28 s over 1 year. Using magnetic data recorded at other locations in a radius of 3000 km it was possible to estimate the diurnal lag and correct the time tamps to produce reliable 1 min averages of magnetic data.

The paper discusses the possibility of improving the quality of geomagnetic variation monitoring at ground observatories. The new fluxgate sensor and electronics with upgraded temperature and noise characteristics are described. It is supposed that the application of the results and recommendations discussed in the paper will allow a fluxgate magnetometer to be created with an outstanding level of parameters.

Chambon-la-Forêt magnetic observatory distributes definitive second data that are contaminated by noise with a standard variation below 400 pT. This noise is low compared to the international standard set by INTERMAGNET. It is mainly due to local signals of unknown origins that have short wavelengths in space (less than 200 m) and in time (less than a day).

In 2013 the scientists from the geophysical centre in Dourbes started a project to install a fully automated observatory in Antarctica. The ultimate aim is to have real-time data coming in to Dourbes every second. After evaluation of different data transfer protocols, we chose to use Message Queuing Telemetry Transport (MQTT) and receive the 1 s data with a negligible latency cost and no loss of data.

The authors developed and built a digital non-magnetic declination–inclination magnetometer which gives all measurement data in digital form. Use of this instrument significantly decreases the possibility of observation errors and minimises handwork. We showed that this device is suitable for absolute magnetic control measurements, and it is more convenient, user friendly and effective than the traditional ones.

Commercial solutions for an automated DI-flux are practically reduced to the AutoDIF and the GyroDIF. We analyze the pros and cons of both in terms of suitability at the Livingston Island geomagnetic observatory, Antarctica. We conclude that the GyroDIF is more suitable for harsh conditions due to its simpler infrastructure. We also show the instrument housing design and its control electronics. Our experiences can benefit the geomagnetic community, which often faces similar challenges.

In geomagnetism, geodesy and in general disciplines requiring orientation on Earth, accurately finding the direction of true north is a challenge. This paper describes a method to do so using a traditional theodolite and the proposed apparatus: an electro-optical add-on. The details of the concepts, design and operation of the add-on are explained.

In this paper, we summarize the difficulties that Sfe detection presents and explain a line of work we initiated to overcome these difficulties with the goal of achieving a system capable of performing automatic detection. Some properties of Sfe, including spherical symmetry around the vortex and different time durations between Sfe and other natural variations, were used to construct an index allowing us to detect Sfe.

In this work, we analysed the Cheongyang geomagnetic baseline data from 2014 to 2016. We observed a step of more than 5 nT in the H and Z baseline in 2014 and 2015 due to artificial magnetic noise in the absolute hut. The baseline also shows a periodic modulation due to temperature variations in the fluxgate magnetometer hut. The quality of the baselines was improved by correcting the discontinuity in the H and Z baselines.

This paper describes the study of the effects of solar wind and solar storms on the national electrical power transmission grid in Austria. These storms result in currents in the ground that can cause damage to power grids, particularly those at high latitudes. Results show that very strong solar storms could result in problems in Austria as well, and this information is important to the grid operators to properly implement mitigation strategies in the future.